With the advance of laser processing machines, magneto-optical devices utilizing the interaction of light and magnetism have recently become of much interest. One of these devices is an isolator, which functions to inhibit the phenomenon that if the light oscillated from a laser source is reflected by the optical system in its path and is returned to the light source, then it disturbs the light oscillated from the laser source, to cause an unstable oscillation state. Accordingly, on use, the optical isolator is arranged between a laser source and an optical member so as to take advantage of the function.
The optical isolator comprises three parts, a Faraday rotator, a polarizer arranged on the light-input side of the Faraday rotator, and an analyzer arranged on the light-output side of the Faraday rotator. The optical isolator utilizes the nature, commonly known as the Faraday effect, that when light enters the Faraday rotator under the condition where a magnetic field is applied to the Faraday rotator in a direction parallel to the light traveling direction, the plane of polarization is rotated in the Faraday rotator. Specifically, the light of the incident light having the same plane of polarization as that of the polarizer is transmitted by the polarizer and enters the Faraday rotator. The light is rotated by +45 degrees relative to the light traveling direction in the Faraday rotator, and then emerges therefrom.
By contrast, when the return light entering the Faraday rotator in an opposite direction to the incident direction first passes through the analyzer, only a component of the light having the same plane of polarization as that of the analyzer is transmitted by the analyzer and enters the Faraday rotator. In the Faraday rotator, the plane of polarization of the return light is further rotated by +45 degrees in addition to the initial +45 degrees. Since the plane of polarization of the return light is at a right angle of +90 degrees with respect to the polarizer, the return light is not transmitted by the polarizer.
It is necessary that the material to be used for the Faraday rotator of the optical isolator mentioned above have a significant Faraday effect and a high transmittance at the wavelength at which it is used.
Also, if a polarized component different from the incident light is generated in the output light, this different polarized component is transmitted by the polarizer, indicating insufficient blockage of the return light.
For the evaluation of the generation of the different polarized component, polarized light of 0 to 90 degrees enters a material used as the Faraday rotator, output light is transmitted by the polarizer into a photodetector, and the intensity of light is measured by the photodetector. From the maximum intensity (Imax) and minimum intensity (Imin), an extinction ratio (S) is computed according to the following equation.S=−10 log(Imin/Imax)(unit: dB)While higher values of extinction ratio are desirable, an extinction ratio of at least 30 dB is generally required.
JP-A 2010-285299 discloses a single crystal oxide of (TbxRe1-x)2O3 (wherein 0.4≦x≦1.0) and transparent oxide ceramics as the material having a high Verdet constant.
As compared with the single crystal oxide, the transparent oxide ceramics are inexpensive and industrially promising because the reaction temperature can be kept low so that large-scale production in a simple plant is possible.
In JP 4033451, for example, a rare earth oxide of the general formula: R2O3 wherein R is a rare earth element is free of birefringence since its crystal structure is cubic. It is described that a sintered body having a high degree of transparency can be produced if pores and impurity segregates are completely removed.
Also, JP-A H05-330913 discloses that addition of sintering aids is effective for removing pores. Further, JP 2638669 discloses removal of pores by hot isostatic pressing and re-sintering.
On the other hand, if heat treatment in the presence of sintering aids is continued for a long time, the sintering aids or the like segregates at grain boundaries. Sometimes, a difference arises between the refractive index of the main phase of crystal grains and the refractive index at the grain boundary.
If there is a difference in refractive index between the main phase and the grain boundary as mentioned above, the polarization state of transmitted light is changed, specifically the extinction ratio becomes lower. An optical isolator manufactured using such material has a poor degree of optical separation.